U.S. Pat. No. 6,686,888 describes a high-frequency antenna for use with a mobile radio that is mounted in the vent grill. The antenna comprises a monopole and ground plane (or balancing antennas) all formed as conductive traces on flat T-shaped printed-circuit board. The board is fitted to the plastic vent grille and acts as a so-called slit antenna, particularly usable in the 2.4 GHz range. The ground plane extends at a right angle to the monopole, and there may be in fact two ground planes.